X-Ray and Optical Studies of Electric Field-Induced Layer Structure Changes in Surface Stabilized Ferroelectric Liquid Crystal Cells.
Surface Stabilized Ferroelectric Liquid Crystals (SSFLC's) are now under development for applications which include optical computing and large flat-screen computer displays. The electro-optic behavior of SSFLC cells is significantly affected by the smectic layer and molecular orientational configurations within the cell. Because an electric field is needed to use these cells, to understand their electro-optic behaviour, we must understand what happens to the layer structure in the presence of an electric field. This thesis describes X-ray and optical experiments determining the smectic layer structure and molecular responses of CS 1014 and ZLI 3654 SSFLC cells to DC and low frequency AC electric fields. At the electric fields typically needed to optically switch SSFLC cells, a continuous, reversible, saturable change of the cell's optic axis has been observed. One model of this behavior proposed reversible smectic layer flexing in SSFLC cells. Our X-ray scattering and spectral transmission measurements indicate that a molecular director response, with unchanging smectic layers, causes this effect. We calculate the optic axis orientational dependence on applied voltage based on the molecular director model and compare it to experimental results. Additionally, we present theoretical arguments against elastic layer flattening in any typical SSFLC cell. The X-ray experiments also yielded the surprising evidence of local layering dislocations within the SSFLC cells examined to date. At higher electric fields, we observe both elastic and plastic layer flexing in electrically driven cells using X-ray scattering; the layer flex changes from elastic to plastic as the incident X-ray intensity increased. All layer flexing appears to occur without an accompanying layer thickness change. This observation weakens previous claims of electric field driven SSFLC elastic layer flex based on X-ray measurements. Without X-rays, the final chevron tilt angle is proportional to the peak past applied electric field. We discuss possible causes for these phenomena. We present the local layer structure of one electric -field induced line defect. The structure, confirmed via X-ray measurements, is that of a locally rotated, flattened chevron structure connected with continuously curving layers to the surrounding chevron. We discuss the electrostatic interactions and possible nucleation mechanisms for the defect. Finally, we discuss the implications of defect observations near the A to C transition, and present several models for layer dislocation nucleation and some experiments to test those models.
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- Physics: Condensed Matter; Chemistry: Polymer; Engineering: Electronics and Electrical